In some drug injection devices, a syringe for storing a drug for injection and an injection needle assembly having an injection needle are separately formed. After the injection needle assembly is attached to a drug discharge tube at a tip of the syringe, the drug injection device may be used to perform an injection.
The injection needle assembly is generally attached to the drug discharge tube with a taper fitting. Specifically, the drug discharge tube is formed to have a male taper shape (a shape of a circular truncated frustum of a cone) in which an outer diameter becomes continuously smaller toward a tip. On the other hand, the injection needle assembly is provided with a fitting portion having a cylindrical hole. This fitting portion is formed to have a female taper shape (a hole with a shape of a circular truncated cone) in which an inner diameter becomes continuously smaller from an opening of the cylindrical hole with a size corresponding to the drug discharge tube. The male taper shape and the female taper shape are formed with the same taper ratio. By applying pressure to the drug discharge tube and pushing the fitting portion, the tapered surfaces rub against each other and are firmly fitted to each other, whereby they are fixed to each other in a liquid-tight manner.
The taper ratio of the taper shape is expressed as a fraction, such as A/100, or as a percentage, such as A %, where, for example, a diameter decreases by A mm per 100 mm.
The International Organization for Standardization specifies taper fittings for medical equipment in ISO594-1: 1986 (Title: Conical Fittings with a 6% (Luer) Taper for Syringes, Needles and Certain Other Medical Equipment—Part 1: General Requirements) and in ISO594-2: 1998 (Title: Conical Fittings with a 6% (Luer) Taper for Syringes, Needles and Certain Other Medical Equipment—Part 2: Lock Fittings). ISO594-1 specifies a type of connection having only a taper fitting, while ISO594-2 specifies a type of connection having a taper fitting with a lock mechanism using a screw. Each standard presupposes that a taper ratio of a taper shape is 6/100.
A drug injection device fixed with a taper fitting is described in, for example, Japanese Patent Application Publication No. 2011-212185 (“Patent Literature 1”). This drug injection device is fixed using a taper fitting with a lock mechanism.
A syringe (cylindrical body) described in Patent Literature 1 has a drug discharge tube (discharge portion) with a male taper shape provided at a tip. A screw portion, serving as a lock mechanism, is provided on an outer peripheral surface of the drug discharge tube at its base. An injection needle assembly (needle hub) described in Patent Literature 1 has a cylindrical hole having a female taper shape at a fitting portion. A screw portion that threadedly engages with the screw portion of the drug discharge tube is formed on the inner peripheral surface of the fitting portion.